The present invention concerns radar-based surveillance systems.
Radar surveillance systems belong to the category of equipment used to detect objects out of the range of human sight. Quite generally, radar equipment, which may be stationary or mobile, is used for a host of purposes such as air and marine traffic control, detection and destruction of hostile objects and platforms such as in air defence, as navigational aids for aircraft and ships, and even for the study of bird flight patterns.
Known radar systems suffer from operational problems in that their functioning is impaired by terrain topography constraints giving rise to so-called hidden areas or dead zones not accessible to a radar beam emitted from a given radar station; and from an intrinsic inability of positive object identification which frequently leads to uncertainty as to the nature of a detected object and consequently to false alarm. Furthermore, as radar systems emit radiation while in operation, they are prone to be detected at an early stage of operation, which in the case of hostilities allows an enemy to take evasive maneuvers, to operate electronic counter-measures, send decoys and launch radiation-homing missiles against the radar station of an air defense system.
The intrinsic insufficiency of radar systems is due to the fact that they display on the radar screen so-called "blips" which are far away from proper pictures and which many times leave the operator perplexed as to what they really represent. In case the detected object is a friendly, manned platform, its nature and identity can be established by interrogation. However, in case of hostile platforms, inanimate objects or non-human living objects such a verification method is not available.
EP 0205794 and the corresponding U.S. Pat. No. 4,780,719 disclose a surveillance system comprising an infrared target data acquisition device operatively coupled to a radar search and tracking device. The infrared data acquisition device does not have any range measurement capability and accordingly, for the range determination on the basis of the elevation and azimuth angle values fed to the radar to be meaningful, the acquisition device must be juxtaposed to the radar device.
U.S. Pat. No. 5,134,409 discloses a surveillance sensor comprising a radar antenna with a co-located and co-rotating electro-optical sensor device, whereby some of the shortcomings of the prior art are allegedly overcome.
Both the above known systems do not take into account the range difference between a radar device, which is of the order of about 200 km, and an optical sensor which as a rule, is of the order of about 30 km only. Accordingly, in surveillance systems which comprise juxtaposed radar and optical sensor devices, the benefit afforded by the optical sensor device can inherently become manifest only at a relatively short range from the radar post, which means that an object will be viewed close to or even over the surveyed area. Clearly, this is a serious disadvantage.